J. Brückner

The Opportunity Rover's Athena science investigation at Meridiani Planum, Mars

The Mars Exploration Rover Opportunity has investigated the landing site in Eagle crater and the nearby plains within Meridiani Planum. The soils consist of fine-grained basaltic sand and a surface lag of hematite-rich spherules, spherule fragments, and other granules. Wind ripples are common. Underlying the thin soil layer, and exposed within small impact craters and troughs, are flat-lying sedimentary rocks. These rocks are finely laminated, are rich in sulfur, and contain abundant sulfate salts. Small-scale cross-lamination in some locations provides evidence for deposition in flowing liquid water. We interpret the rocks to be a mixture of chemical and siliciclastic sediments formed by episodic inundation by shallow surface water, followed by evaporation, exposure, and desiccation. Hematite-rich spherules are embedded in the rock and eroding from them. We interpret these spherules to be concretions formed by postdepositional diagenesis, again involving liquid water.

Chemistry of Rocks and Soils at Meridiani Planum from the Alpha Particle X-ray Spectrometer

The Alpha Particle X-ray Spectrometer on the Opportunity rover determined major and minor elements of soils and rocks in Meridiani Planum. Chemical compositions differentiate between basaltic rocks, evaporite-rich rocks, basaltic soils, and hematite-rich soils. Although soils are compositionally similar to those at previous landing sites, differences in iron and some minor element concentrations signify the addition of local components. Rocky outcrops are rich in sulfur and variably enriched in bromine relative to chlorine. The interaction with water in the past is indicated by the chemical features in rocks and soils at this site.

An integrated view of the chemistry and mineralogy of martian soils

The mineralogical and elemental compositions of the martian soil are indicators of chemical and physical weathering processes. Using data from the Mars Exploration Rovers, we show that bright dust deposits on opposite sides of the planet are part of a global unit and not dominated by the composition of local rocks. Dark soil deposits at both sites have similar basaltic mineralogies, and could reflect either a global component or the general similarity in the compositions of the rocks from which they were derived. Increased levels of bromine are consistent with mobilization of soluble salts by thin films of liquid water, but the presence of olivine in analysed soil samples indicates that the extent of aqueous alteration of soils has been limited. Nickel abundances are enhanced at the immediate surface and indicate that the upper few millimetres of soil could contain up to one per cent meteoritic material.

Mineralogic and compositional properties of Martian soil and dust: Results from Mars Pathfinder

Mars Pathfinder obtained multispectral, elemental, magnetic, and physical measurements of soil and dust at the Sagan Memorial Station during the course of its 83 sol mission. We describe initial results from these measurements, concentrating on multispectral and elemental data, and use these data, along with previous Viking, SNC meteorite, and telescopic results, to help constrain the origin and evolution of Martian soil and dust. We find that soils and dust can be divided into at least eight distinct spectral units, based on parameterization of Imager for Mars Pathfinder (IMP) 400 to 1000 nm multispectral images. The most distinctive spectral parameters for soils and dust are the reflectivity in the red, the red/blue reflectivity ratio, the near-IR spectral slope, and the strength of the 800 to 1000 nm absorption feature. Most of the Pathfinder spectra are consistent with the presence of poorly crystalline or nanophase ferric oxide(s), sometimes mixed with small but varying degrees of well-crystalline ferric and ferrous phases. Darker soil units appear to be coarser-grained, compacted, and/or mixed with a larger amount of dark ferrous materials relative to bright soils. Nanophase goethite, akaganeite, schwertmannite, and maghemite are leading candidates for the origin of the absorption centered near 900 nm in IMP spectra. The ferrous component in the soil cannot be well-constrained based on IMP data. Alpha proton X-ray spectrometer (APXS) measurements of six soil units show little variability within the landing site and show remarkable overall similarity to the average Viking-derived soil elemental composition. Differences exist between Viking and Pathfinder soils, however, including significantly higher S and Cl abundances and lower Si abundances in Viking soils and the lack of a correlation between Ti and Fe in Pathfinder soils. No significant linear correlations were observed between IMP spectral properties and APXS elemental chemistry. Attempts at constraining the mineralogy of soils and dust using normative calculations involving mixtures of smectites and silicate and oxide minerals did not yield physically acceptable solutions. We attempted to use the Pathfinder results to constrain a number of putative soil and dust formation scenarios, including palagonitization and acid-fog weathering. While the Pathfinder soils cannot be chemically linked to the Pathfinder rocks by palagonitization, this study and McSween et al. [1999] suggest that palagonitic alteration of a Martian basaltic rock, plus mixture with a minor component of locally derived andesitic rock fragments, could be consistent with the observed soil APXS and IMP properties.

Water alteration of rocks and soils on Mars at the Spirit rover site in Gusev crater.

Gusev crater was selected as the landing site for the Spirit rover because of the possibility that it once held a lake. Thus one of the rovers tasks was to search for evidence of lake sediments. However, the plains at the landing site were found to be covered by a regolith composed of olivine-rich basaltic rock and windblown ‘global’ dust. The analyses of three rock interiors exposed by the rock abrasion tool showed that they are similar to one another, consistent with having originated from a common lava flow. Here we report the investigation of soils, rock coatings and rock interiors by the Spirit rover from sol (martian day) 1 to sol 156, from its landing site to the base of the Columbia hills. The physical and chemical characteristics of the materials analysed provide evidence for limited but unequivocal interaction between water and the volcanic rocks of the Gusev plains. This evidence includes the softness of rock interiors that contain anomalously high concentrations of sulphur, chlorine and bromine relative to terrestrial basalts and martian meteorites; sulphur, chlorine and ferric iron enrichments in multilayer coatings on the light-toned rock Mazatzal; high bromine concentration in filled vugs and veins within the plains basalts; positive correlations between magnesium, sulphur and other salt components in trench soils; and decoupling of sulphur, chlorine and bromine concentrations in trench soils compared to Gusev surface soils, indicating chemical mobility and separation.

Chemical, multispectral, and textural constraints on the composition and origin of rocks at the Mars Pathfinder landing site

Rocks at the Mars Pathfinder site are probably locally derived. Textures on rock surfaces may indicate volcanic, sedimentary, or impact-generated rocks, but aeolian abration and dust coatings prevent unambiguous interpretation. Multispectral imaging has resolved four spectral classes of rocks: gray and red, which occur on different surfaces of the same rocks; pink, which is probably soil crusts; and maroon, which occurs as large boulders, mostly in the far field. Rocks are assigned to two spectral trends based on the position of peak reflectance: the primary spectral trend contains gray, red, and pink rocks; maroon rocks constitute the secondary spectral trend. The spatial pattern of spectral variations observed is oriented along the prevailing wind direction. The primary spectral trend arises from thin ferric coatings of aeolian dust on darker rocks. The secondary spectral trend is apparently due to coating by a different mineral, probably maghemite or ferrihydrite. A chronology based on rock spectra suggests that rounded maroon boulders constitute the oldest petrologic unit (a flood deposit), succeeded by smaller cobbles possibly deposited by impact, and followed by aeolian erosion and deposition. Nearly linear chemical trends in alpha proton X-ray spectrometer rock compositions are interpreted as mixing lines between rock and adhering dust, a conclusion supported by a correlation between sulfur abundance and red/blue spectral ratio. Extrapolations of regression lines to zero sulfur give the composition of a presumed igneous rock. The chemistry and normative mineralogy of the sulfur-free rock resemble common terrestrial volcanic rocks, and its classification corresponds to andesite. Igneous rocks of this composition may occur with clastic sedimentary rocks or impact melts and breccias. However, the spectral mottling expected on conglomerates or breccias is not observed in any APXS-analyzed rocks. Interpretation of the rocks as andesites is complicated by absence of a “1 μm” pyroxene absorption band. Plausible explanations include impact glass, band masking by magnetite, or presence of calcium- and iron-rich pyroxenes and olivine which push the absorption band minimum past the imagers spectral range. The inferred andesitic composition is most similar to terrestrial anorogenic icelandites, formed by fractionation of tholeiitic basaltic magmas. Early melting of a relatively primitive Martian mantle could produce an appropriate parent magma, supporting the ancient age of Pathfinder rocks inferred from their incorporation in Hesperian flood deposits. Although rocks of andesitic composition at the Pathfinder site may represent samples of ancient Martian crust, inferences drawn about a necessary role for water or plate tectonics in their petrogenesis are probably unwarranted.

The Mars Odyssey Gamma-Ray Spectrometer Instrument Suite

The Mars Odyssey Gamma-Ray Spectrometer is a suite of three different instruments, a gamma subsystem (GS), a neutron spectrometer, and a high-energy neutron detector, working together to collect data that will permit the mapping of elemental concentrations on the surface of Mars. The instruments are complimentary in that the neutron instruments have greater sensitivity to low amounts of hydrogen, but their signals saturate as the hydrogen content gets high. The hydrogen signal in the GS, on the other hand, does not saturate at high hydrogen contents and is sensitive to small differences in hydrogen content even when hydrogen is very abundant. The hydrogen signal in the neutron instruments and the GS have a different dependence on depth, and thus by combining both data sets we can infer not only the amount of hydrogen, but constrain its distribution with depth. In addition to hydrogen, the GS determines the abundances of several other elements. The instruments, the basis of the technique, and the data processing requirements are described as are some expected applications of the data to scientific problems.

Overview of the Spirit Mars Exploration Rover Mission to Gusev Crater: Landing site to Backstay Rock in the Columbia Hills

Spirit landed on the floor of Gusev Crater and conducted initial operations on soil-covered, rock-strewn cratered plains underlain by olivine-bearing basalts. Plains surface rocks are covered by wind-blown dust and show evidence for surface enrichment of soluble species as vein and void-filling materials and coatings. The surface enrichment is the result of a minor amount of transport and deposition by aqueous processes. Layered granular deposits were discovered in the Columbia Hills, with outcrops that tend to dip conformably with the topography. The granular rocks are interpreted to be volcanic ash and/or impact ejecta deposits that have been modified by aqueous fluids during and/or after emplacement. Soils consist of basaltic deposits that are weakly cohesive, relatively poorly sorted, and covered by a veneer of wind-blown dust. The soils have been homogenized by wind transport over at least the several kilometer length scale traversed by the rover. Mobilization of soluble species has occurred within at least two soil deposits examined. The presence of monolayers of coarse sand on wind-blown bedforms, together with even spacing of granule-sized surface clasts, suggests that some of the soil surfaces encountered by Spirit have not been modified by wind for some time. On the other hand, dust deposits on the surface and rover deck have changed during the course of the mission. Detection of dust devils, monitoring of the dust opacity and lower boundary layer, and coordinated experiments with orbiters provided new insights into atmosphere-surface dynamics.

Chemical Composition of Rocks and Soils at the Pathfinder Site

As Viking Landers did not measure rock compositions, Pathfinder (PF) data are the first in this respect. This review gives no proof yet whether the PF rocks are igneous or sedimentary, but for petrogenetic reasons they could be igneous. We suggest a model in which Mars is covered by about 50% basaltic and 50% andesitic igneous rocks. The soils are a mixture of the two with addition of Mg-sulfate and -chloride plus iron compounds possibly derived from the hematite deposits.

Overview of the Mars Pathfinder Mission: Launch through landing, surface operations, data sets, and science results

Mars Pathfinder successfully landed at Ares Vallis on July 4, 1997, deployed and navigated a small rover about 100 m clockwise around the lander, and collected data from three science instruments and ten technology experiments. The mission operated for three months and returned 2.3 Gbits of data, including over 16,500 lander and 550 rover images, 16 chemical analyses of rocks and soil, and 8.5 million individual temperature, pressure and wind measurements. Path-finder is the best known location on Mars, having been clearly identified with respect to other features on the surface by correlating five prominent horizon features and two small craters in lander images with those in high-resolution orbiter images and in inertial space from two-way ranging and Doppler tracking. Tracking of the lander has fixed the spin pole of Mars, determined the precession rate since Viking 20 years ago, and indicates a polar moment of inertia, which constrains a central metallic core to be between 1300 and ∼2000 km in radius. Dark rocks appear to be high in silica and geochemically similar to anorogenic andesites; lighter rocks are richer in sulfur and lower in silica, consistent with being coated with various amounts of dust. Rover and lander images show rocks with a variety of morphologies, fabrics and textures, suggesting a variety of rock types are present. Rounded pebbles and cobbles on the surface as well as rounded bumps and pits on some rocks indicate these rocks may be conglomerates (although other explanations are also possible), which almost definitely require liquid water to form and a warmer and wetter past. Air-borne dust is composed of composite silicate particles with a small fraction of a highly magnetic mineral, interpreted to be most likely maghemite; explanations suggest iron was dissolved from crustal materials during an active hydrologic cycle with maghemite freeze dried onto silicate dust grains. Remote sensing data at a scale of a kilometer or greater and an Earth analog correctly predicted a rocky plain safe for landing and roving with a variety of rocks deposited by catstrophic floods, which are relatively dust free. The surface appears to have changed little since it formed billions of years ago, with the exception that eolian activity may have deflated the surface by ∼3–7 cm, sculpted wind tails, collected sand into dunes, and eroded ventifacts (fluted and grooved rocks). Pathfinder found a dusty lower atmosphere, early morning water ice clouds, and morning near-surface air temperatures that changed abruptly with time and height. Small scale vortices, interpreted to be dust devils, were observed repeatedly in the afternoon by the meteorology instruments and have been imaged.